Construction Fact Sheet - Printable Version (1.2MB)
Planning and environmental analysis studies for the Westside Subway Extension (Project) were initiated by Metro and the Federal Transit Administration in late 2007. This effort began with the initial Alternatives Analysis Study (AA) in 2007-08 and was followed in 2009-10 with the Draft Environmental Impact Statement/Environmental Impact Report (Draft EIS/EIR). Metro is currently preparing the Final EIS/EIR. These combined efforts have included in-depth study and analysis of various transportation modes, alignments and many other technical issues. As the environmental process is concluding, and with construction potentially beginning as early as 2013, this fact sheet is intended to summarize the subway construction process.
There are two basic elements of subway construction – building the stations and their entrances, and building the tunnels running between the stations. Stations and tunnels are constructed in very different ways. Stations are constructed from the surface by excavating the area to be occupied by the station “box.” Construction staging areas are usually located adjacent to station construction sites. Tunnels are excavated below ground using tunnel boring machines (TBMs).
The greatest construction impacts will likely be generated by station construction and from the associated construction staging sites. Little disturbance on the surface is likely from the tunnel construction between the station areas. The Final EIS/EIR will evaluate impacts associated with construction and propose mitigations for these impacts.
This fact sheet provides an overview of the construction process for stations & tunnels, and also discusses construction staging areas and possible construction impacts.
The station is a large box, usually about 600-700 feet long and 60 feet wide. In some cases, the box must be longer when crossover tracks are appended to the end of the station. In these cases, the box is about 1,000 feet long. Crossover tracks are required at end stations to turn trains around and periodically allow trains to cross to the opposite track if needed to pass a stalled train or maintenance zone.
When completed, the box will accommodate:
Stations are generally 50-60 feet deep to allow easy passenger circulation from the station platform to the surface. When built under a street, they are constructed below temporary concrete decking that allows the street to continue to carry traffic. If located off-street, they can be built using an open excavation, similar to the construction of a building.
Initial street excavation for stations may require temporary lane or street closures.
Constructing a station is a multi-step process and somewhat more complex if the station is being constructed under a street. Preparing a site for station construction typically begins by protecting or relocating any underground utilities such as power lines, water lines, sewers, gas pipes, cable/telephone lines and storm drains. This will likely require temporary closures of portions of the street under which utilities are located and detouring traffic around the work site. Detours are often limited to weekends or non-peak periods.
The next step in the process is to install concrete decking that will serve as the temporary street surface, allowing traffic to continue to flow while construction continues underneath. Vertical support piles are installed along the edges of the street and steel beams are installed across the construction area atop these piles. Concrete decking is then installed in sections on top of the beams flush with the street. The temporary decking is also designed to maintain access to sidewalks and driveways, wherever possible.
This initial street excavation and installation of the concrete decking requires temporary street closures. This typically occurs over a series of sequential weekends beginning after rush hour on Friday night, with the street reopening before rush hour on Monday morning. In some cases, communities may prefer to close the streets continuously to shorten the overall duration of this process. If the work can be accomplished by closing only a portion of the lanes, traffic will be accommodated in the remaining lanes. In some instances, the entire street may need to be closed for some period of time. If so, traffic will be temporarily diverted to parallel streets.
Once the concrete decking is in place, the traffic continues to flow above while station construction continues below.
The next steps involve removing the earth within the area that will eventually become the station box. At the same time, shoring is installed along the edges of the excavation to support the ground around the station box.
Once excavation is completed to the bottom of the station box, construction of the inside of the station begins.
The public areas of the subway stations also contain architectural design treatments and art work, information displays, lighting, signage, security monitoring devices and many other design elements.
One of the final steps in the process is the removal of the decking and restoration of the street. This can again be done at night and on weekends, or over a shorter period of time by closing the street continuously and rebuilding the street on top of the station box. When construction is finished, there is little evidence on the surface other than the station entrances.
Station construction is estimated to take five to seven years.
Tunnels are typically about 20 feet in diameter. There are two parallel tunnels separated by about 20 feet, one tunnel for train travel in each direction. Tunnels are generally about 50-70 feet deep, though they can be deeper between stations. Some portions of the tunnel alignments currently being planned for the Project are significantly deeper than this, up to about 130 feet.
The twin tunnels between stations are constructed with TBMs that must be lowered into the ground by cranes through a large shaft referred to as the TBM launch site. These sites are typically located in a staging area near one or more of the station boxes. The TBMs proceed in parallel, tunneling at an average rate of 40 to 50 feet per day.
The cutting face and other aspects of TBMs are typically custom-made for each tunneling job depending on the soil conditions that will be encountered. However most urban tunneling around the world today utilizes what is known as pressurized-faced TBMs. These machines maintain the pressure in the surrounding ground, and precast concrete linings are installed as the machines progress. Most recently, Metro used pressurized-face TBMs on the 1.8 mile tunnel for the Metro Gold Line Eastside Extension and experienced no measurable surface subsidence or substantiated property damage claims. Another recent project utilizing pressurized-face TBMs is the City of Los Angeles’ large diameter East Central Interceptor Sewer which runs under portions of Exposition Bl.
Different measures can be built into the tunnels along the way to accommodate special conditions. In areas with gassy or watery ground this could include secondary tunnel liners or gaskets, or enhanced ventilation systems. In areas where the tunnel may cross an earthquake fault, a wider tunnel may be excavated or flexible tunnel liners could be installed.
As the tunneling progresses, the excavated material is brought back to the TBM launch site through the tunnel using bins mounted on rail cars or a conveyor system. Alternately, some systems use a slurry transport method to remove soil to the surface. With a slurry system, excavated soil is mixed with a fluid so it can be pumped thru pipelines in the tunnel. Soil will then be separated from the fluid at the surface worksite. Once the tunneling between the stations is completed and the tunnel surface is finished, the tracks and electrical facilities can be installed.
Construction Staging Areas
While most of the construction activity takes place below ground, there is also the need for a significant amount of space at the surface to store materials and stage construction activities. It is preferable to utilize two staging areas directly adjacent to each station to allow access to the station construction area to expedite the process. The combined staging area at each station is typically about one acre in size. At locations where TBMs are launched and/or the earth from the tunneling process will be removed, a larger staging area is desired. This larger staging area of about three acres will include the earth removal location, areas to temporarily store the earth and potentially sort it for appropriate disposal, areas for off-street truck loading and unloading, and equipment/construction material storage. The staging areas may also include construction trailers for offices and workshops and some employee parking. Often, the construction staging area is also the site where the station entrance (portal) will be located. The staging areas may be on property purchased by Metro or leased from private property owners for the construction period. Owners who retain ownership can then develop their property once subway construction is complete. See the Property Acquisition Fact Sheet for more information.
Construction timing for the Project is dependent upon how the funding package for the project comes together. Presuming that the environmental clearance process concludes in 2011 and funding is secured, final design and contractor selection processes would occur in 2012. It is likely that early utility relocation work and removal of paleontological resources (fossils) below Wilshire Bl in the vicinity of the La Brea Tar Pits could start sometime in 2012, with heavier construction starting on tunnels and stations in 2013. If funding is secured to build the 9-mile extension all at the same time, construction along the entire alignment to the Westwood/VA Hospital could potentially be completed by 2022. In this case, several pairs of TBMs would be used, tunneling various segments of tunnel at the same time, with work proceeding on all stations simultaneously. If the project must be built in phases, based on the adopted Metro Long Range Transportatation, the first phase would proceed west from the Wilshire/Western Station to approximately La Cienega and it could take until 2036 to complete subsequent construction phases to reach the Westwood/VA station.
Determining the Construction Impacts and Mitigations
Clearly, subway construction cannot be accomplished without impacts. Some of the impacts from subway construction could be:
When the Final EIS/EIR for the Project is released, it will provide information about how the subway will be built including impacts from the construction process. These include any potential impacts resulting from constructing the stations, station portals, the use of construction staging locations, and the underground tunnels.
The Final EIS/EIR will also propose mitigations to eliminate or reduce any of these impacts. Possible mitigations may include:
The public is encouraged to review the Final EIS/EIR document for information about construction impacts and proposed mitigations. Following public release of the Final EIS/EIR, it will be presented to the Metro Board of Directors. They will be asked to adopt the project that will be built including project mitigations.
More information about subway construction is available on our website metro.net/westside. You may wish to view our community update presentations from January 2011 and August 2009 that focused on subway construction.
How to Stay Involved and Give Input
Metro invites you to stay involved throughout the study. You can keep up with developments on our website, metro.net/westside, where you can find information as work progresses, leave comments, and let us know how to contact you so we can keep you informed of upcoming meetings and other milestones.
One Gateway Plaza, 99-22-5
Los Angeles, CA 90012
Comments may be submitted by:
Online: Complete our electronic Comment/Question Form
By U.S. Mail:
David Mieger, Project Director
DEO, Countywide Planning & Development
Los Angeles Metropolitan Transportation Authority
1 Gateway Plaza, M/S 99-22-5
Los Angeles, CA 90012
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